The 1950-D over S Quarter Dollar is one of the more popular varieties in the series. Mint State examples, while not common, appear on the market fairly frequently in grades up to MS65. In MS66 and better, this variety is very rare; the finest example certified by PCGS as of 2/9/2011 is a single MS67.
On this variety, the mintmark looks like a D over D over S, with the top and left of the S appearing underneath the upper left side of the final D. This variety is also known as FS-601 (old FS-021).
Sources and/or recommended reading:
"Cherrypickers' Guide to Rare Die Varieties of United States Coins, 4th Edition, Volume II" by Bill Fivaz and J.T. Stanton